Well it really depends. With your card I would really only recommend 2x.
2x really is the sweet spot, since every step up from there is less noticable.
Crysis has it own type of edge smoothing technique too.
normal quality is r_useedgeaa 1
High quality is r_useedgeaa 2
They overall don't look as good, but the difference is, is that they apply to foliage, and they are much more efficient.